The route is 134.734 miles (216.833 km) long, and it traverses Barren, Metcalfe, Cumberland, Clinton, Wayne, Pulaski, McCreary and Whitley Counties in southern Kentucky.

It is a major tourism hot-spot due to the close proximity to Mammoth Cave National Park, in nearby Edmonson County, along with few other attractions along KY 70 west of the city.

[4] It turns due east, crosses the Cumberland River, and has intersections with roads leading to Dale Hollow Lake State Park, and then goes into Clinton County, where, for about 1.5 miles (2.4 km), it is co-joined with US 127, which is the core route of the annual World's Longest Yard Sale (a.k.a.

[6] After entering Pulaski County it becomes a four-lane highway, crosses Lake Cumberland and turns south on a concurrency with US 27 from Burnside, Kentucky.

Early in this stretch of road is the gateway to General Burnside State Park; it later enters the Daniel Boone National Forest.

[7] After departing US 27 in northern McCreary County,[8] KY 90 is the main route that accesses Cumberland Falls State Resort Park.

[9] At the route's terminus in the Whitley County community of Young's Creek, there is a 0.32 mile-long (0.51 km) spur called KY 90S that serves as a secondary entrance to the highway for traffic going southbound on U.S. 25W.

The highway's original western terminus was located in downtown Glasgow at a junction with KY 80 (which at the time was not co-signed with US 68 in this area until the 1940s).

[15] Sometime between 1940 and 1955, US 25W was rerouted to the west, to put it closer to Cumberland Falls, and that gave it a concurrency with KY 90's eastern end that was abolished by 1957.

[15][18] Sometime between 1969 and 1972, following the completion of I-65, KY 90 was improved from Glasgow and got a new route directly into Cave City, east of Prewitt's Knob.

[19] In 2013, KY 90 between the Cumberland Parkway junction and the Eighty Eight community was reconstructed to add truck-climbing lanes in two spots.